As computer systems have advanced, the instructions-per-second rate has dramatically increased. Disk drive performance has not been able keep up with computer technological progress. I/O bottlenecking results from a disparity between computer system speed and disk access speed. Another concern with disk storage is its reliability. In 1987, a system was introduced to alleviate the concerns of I/O performance and data reliability. This system, called RAID, for Redundant Array of Inexpensive Disks, consists of several methods of writing data to an array of common disks. The third method, called RAID-3, stripes (i.e., interleaves) words of data across the parallel array of drives as well as stores parity data on the array. RAID-3 is useful for transfers of larger blocks of data. The fifth method, called RAID-5, stripes sectors of data across the drives. RAID-5, like RAID-3 stores parity data, however on a RAID-5 system, the parity data is interleaved among all the drives rather than being written to a disk drive dedicated to parity data. See, David A. Patterson, Garth Gibson, Randy H. Katz, "A Case for Redundant Arrays of Inexpensive Disks (RAID)," University of California at Berkeley, Report No. UCB/CSD 87/391, December 1987! The parallellism of the disk array allows writes to be done simultaneously, thus the increase in disk performance.
A parallel array of disk drives has disadvantages. The primary disadvantage is that each disk drive requires a separate I/O channel. Thus, for an array of five disk drives (called a 4+1 array, indicating 4 drives dedicated to data storage and one for parity), five disk drive I/O channels must be implemented. The need for a separate channel for each disk drive increases the cost of implementing RAID systems. What is needed is a way to get the benefits of a disk array without the cost of allocating a channel to each disk drive.